As disclosed in Japanese Patent Application Laid-Open Publication No. 117873/2003 and Japanese Patent Application Laid-Open Publication No. 175484/2003, as a conventional mechanism used for a robot arm or the like, a link mechanism which rotates two links through a rotary joint mechanism having a real axis of rotation is known.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 117873/2003    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 175484/2003